It suggests in the Guideline that the postmenopausal women with breast cancer who have taken selective estrogen receptor modulators (SERMs) therapy for 2-3 years could benefit from changing endocrine therapy to aromatase inhibitors (AIs). This is a prospective, randomized and non-inferior trial to evaluate the prognosis of changing endocrine therapy from SERMs to AIs in perimenopausal and recently postmenopausal women with early-stage hormone receptor-positive breast cancer.